The present invention generally relates to display devices, methods for manufacturing the display devices and display panels for use therein and, more particularly, to a display device which is suitable for liquid crystal display, a manufacturing method and a display panel.
There has been a tendency that a liquid crystal display device is made larger in screen size and higher in definition. The former requires a substrate itself to be made larger, while the latter requires the reduction of pixel size and processing dimensions. The larger-scale and higher-definition display device involves technically increased difficulties and remarkable reduction of its yield in manufacturing the display device. In an attempt to increase the display yield redundancy is provided to the constituent elements of the display device, as disclosed, for example, in JP-A-57-49997. However, a method for incorporating a driver circuit in a peripheral area of a display device to reduce its cost as disclosed in JP-A-59-10988, in particular, has a problem that, since a defect in the peripheral circuit causes appearance of a line defect on the display screen, it becomes very important to improve the yield of the peripheral circuit. In the above prior art technique of JP-A-57-49997 for providing redundancy, the redundancy application is used mainly for the display area, i.e., pixel area of the display device but it is also used for improving the yield of the peripheral circuit. However, the formation of the peripheral circuit usually requires a high temperature process when compared with the formation of the pixel area, thus blocking the realization of a high yield.
In a display device of JP-A-63-223788, for the purpose of improving the characteristics of a peripheral circuit incorporated therein, a scan drive circuit is made of amorphous silicon while a signal drive circuit is made of polycrystalline silicon. However, any consideration is not paid to the improvement of the overall yield of the display device.
In JP-A-1-45162, in order to enhance the yield of a display device incorporating a peripheral circuit, a device is taken wherein a photo-lithography process is used to separate a silicon film prior to a laser annealing process, but no consideration is paid to the structure of the display device.
As a prior art TFT (thin film transistor) panel for use in a liquid crystal display device, for example, JP-A-64-2088 or JP-A-60-26932 disclose an active matrix panel which incorporates a peripheral circuit and wherein TFT elements for respective pixels and the peripheral circuit for driving the TFT elements are formed on the same substrate.
Further known as in JP-A-63-186216 or JP-A-61-121034 is another large-scale TFT panel wherein a plurality of TFTs are positioned within each of pixels for the purpose of applying redundancy to the large-scale panel and improving the yield of the panel.
JP-A-61-180275 also discloses a method for manufacturing a large-scale TFT panel which employs a divisional light exposure process. In this prior art, however, no consideration is paid to making full use of the respective characteristics of the TFTs for the pixel area and the peripheral circuit area.
In the prior art techniques, in addition, individual considerations have been paid to the yield improvement of the pixel area and the yield improvement of the peripheral circuit, but such consideration has not been sufficiently taken of how to improve the yield taking into account the differences in characteristic and manufacturing process between the peripheral circuit and pixel area and also taking into account the influences of the differences in characteristic and manufacturing process between the pixel area, scan drive circuit and data drive circuit on the arrangement of the display device.